Field of the Invention
The present disclosure relates to a multistage compressor and, more particularly, to a multistage compressor with a cooling unit including a cooling fan controlled by temperature sensors located between compression stages of the multistage compressor.
Description of Related Art
Mechanical single stage, air compressors are well known in the art. Such single stage compressors include a driving mechanism for compressing air contained within a compression chamber, such as piston and cylinder, centrifugal, axial-flow, or turbine type mechanism. The simplest and most common mechanisms in use is the piston and cylinder arrangement. In this type, a gas, such as air, is admitted via a valve into the cylinder where a reciprocating piston in the cylinder compresses the gas and displaces the compressed gas to a conduit or reservoir from which it can be taken for use as may be required. For example, compressed gas may be used for operating brakes of a rail vehicle.
Multiple stage air compressors are also known. Such multiple stage compressors are utilized to compress gasses to pressures which are higher than can normally be achieved with a single stage compressor. These multiple stage compressors normally include a plurality of mechanical single stage compressors connected to one another in series, such that compressed gas is passed from one stage to the next. The pressure of the gas increases at each succeeding stage. In a piston and cylinder type compressor, air or gas, at ambient pressure and temperature, is admitted into the cylinder of the first compressor stage where a first reciprocating piston compresses the gas and displaces it to the second stage, and so on through all the stages in the system. Each stage further compresses the previously compressed gas until the final desired pressure is achieved.
Multistage compressors generally include cooling steps of the compressed gas between at least some of the various compressor stages. Cooling the compressed gas between stages ensures that the overall compression is more isothermal than adiabatic. More specifically, because of the ideal gas law, (PV=nRT), each compression stage of the gas will cause an increase in pressure, P, as intended, and will also cause a directly proportional increase in the temperature of the fluid.
While this increase in temperature of the gas is not normally a problem in a typical single stage compressor where a defined volume of air is compressed only one time, the relatively high air pressures obtained in most multistage compressors can result in the compressed air having excessive and problematical temperatures. Accordingly, it is necessary to perform the intermediate cooling step of the compressed gas between the various compression stages. The intermediate cooling step can be performed by a cooling element, referred to as an intercooler, such as a radiator or heat exchanger.
Water exists as vapor in practically any ambient air to be compressed in a conventional compressor. Water content is quantified as the relative humidity of the air. The relative humidity of the air, expressed as a percent value, is the ratio of (a) the water vapor actually present in the air, in comparison to (b) the saturation vapor pressure at the temperature in question. Since the saturation vapor pressure is a function of the air temperature, as the temperature increases for any given sample of air, the saturation vapor pressure increases, and, accordingly, the relative humidity decreases. When the air is compressed by an air compressor with little or no externally caused change in temperature, the temperature of the compressed air is increased in proportion to the increase in pressure. Because the saturation vapor pressure of water is dependent on the temperature of the air, it follows that when the temperature is increased the saturation vapor pressure is also increased.
Thereafter, if the compressed air is cooled with an intercooler, it is not uncommon for the water vapor pressure in the compressed air to exceed the saturation vapor pressure for the compressed air. Therefore, it is not uncommon for this phenomenon to cause significant amounts of water to be condensed as liquid within the system. Free water within the compressor causes a variety of problems, such as oxidation (e.g., rusting) of compressor components, and more significantly, causes condensed water to be admixed into the lubricating oil within the compressor sump. Such dilution of the lubricating oil in the compressor with water can seriously impair the normal operation of the compressor, as well as reduce its overall useful life. For example, rust formation can cause valve leakage. Additionally, water traveling at high velocities through valves of the compressor can cause wear, leakage, and eventual failure of valve components. Therefore, it is desirable to substantially minimize or eliminate the condensation of such water within any compressor.
Multistage compressors often also include an additional cooling step, referred to as an aftercooler. In the aftercooler, discharge from the final compression stage is cooled to a temperature close to ambient temperature before it exits the compressor. The aftercooler functions to decrease the temperature of the compressed air to allow a maximum amount of entrapped water vapor to condense to water form before being discharged from the compressor. The water can be collected and drained from the compressor. Optionally, the compressed and cooled air may be passed through an inline air dryer to remove remaining water vapor therefrom. Cooling the air prior to passing it through the inline air dryer has been found to improve the operation and efficiency of the air dryer, which also improves downstream air quality.
However, in ambient conditions that are below freezing, the water condensation within the aftercooler can freeze within the aftercooler or before it is removed from the compressor through the drain. Airline freeze can block fluid from exiting the compressor and render the compressor unusable. Blockages within the air line can also cause unsafe over-pressurization within systems that are not properly protected by safety valves. Therefore, it is highly desirable to maintain the temperature of the discharged air above freezing, which also reduces water condensation at the discharge of the aftercooler.
In order to prevent condensation and/or freezing of condensed fluid within the compressor, various arrangements or mechanisms for controlling the temperature of gas passing through a cooling element, such as an intercooler or aftercooler, are known. For example, U.S. Pat. No. 5,885,060 to Cunkelman et al. (hereinafter “the Cunkelman patent”) and U.S. Pat. No. 6,283,725 to Goettel et al. (hereinafter “the Goettel patent”), each of which are incorporated by reference herein in its entirety, disclose multistage compressors that use various bypass arrangements to allow a portion of the compressed gas to bypass the intercooler or aftercooler. The bypassed gas is mixed with gas cooled by the intercooler or aftercooler at the intercooler or aftercooler discharge to warm the cooled gas. The bypass arrangement may be optimized to ensure that the mixed gas is maintained within a specific temperature range that is optimal for compressor operation. In that case, the arrangement may include a controller and a switch or valve for ensuring that the correct amount of gas enters the bypass to ensure that the desired temperature is maintained at the intercooler or aftercooler discharge.
More specifically, the Cunkelman patent discloses a thermostatically controlled intercooler system that prevents condensation of a gas within the compressor. The system includes a three-way valve controlled by a controller unit. The controller unit operates the three-way valve to allow cooled air, un-cooled air, or a combination thereof to pass to a second or subsequent compression stage. The valve may include a built-in temperature control that operates so that air at the cooler discharge is maintained within a desired temperature range. The Goettle patent discloses an aftercooler bypass arrangement in which uncooled air is directed around the aftercooler and mixed with cooled air at a discharge of the cooler. The bypass arrangement includes a three-way check valve, as described above. The value is controlled so that the temperature of air at the aftercooler discharge is maintained at or above freezing temperature for the fluid being compressed.
However, such air bypass and mixing arrangements are needlessly complex, requiring dedicated conduits or fluid channels for directing air flow around the cooling elements. Furthermore, since most multistage compressors include both an intercooler and an aftercooler, it is generally necessary to include dedicated and independent bypass arrangements for each cooling element used in the compressor. Accordingly, there is a need for a simpler system or method for controlling air temperature between compression stages of a multistage compressor which controls the temperature at a discharge of the intercooler and the aftercooler using the same device or mechanism. Furthermore, the arrangement or system should be able to integrate with existing elements of a compressor system. The multistage compressor and method described hereinafter is intended to address and improve these issues.